1. Field
The present disclosure relates to a resin-framed membrane electrode assembly.
2. Description of the Related Art
Fuel cells that cause electrochemical reaction of reactant gas so as to generate electric power have attracted attention as new power sources for recent automobiles. The fuel cells directly obtain electricity through electrochemical reaction, and thus, are considered preferable in view of high power generation efficiency. In addition, fuel cells generate only harmless water during electric power generation, and thus, are also considered preferable in view of environmental effects.
A solid polymer electrolyte fuel cell, for example, has a stack structure in which several tens to several hundreds of cells are stacked. In each cell, a membrane electrode assembly (MEA) is held between a pair of separators. The membrane electrode assembly includes an anode (a negative electrode), a cathode (a positive electrode), and an electrolyte membrane held between the anode and the cathode. Each of the anode and the cathode includes a catalyst layer that is in contact with the electrolyte membrane and a diffusion layer that is in contact with the catalyst layer. The separator has one surface provided with a fuel gas channel and another surface provided with an oxidizing agent gas channel.
In the solid polymer electrolyte fuel cell with the configuration as described above, hydrogen serving as fuel gas is supplied to the anode through the fuel gas channel. In addition, air serving as oxidizing agent gas is supplied to the cathode through the oxidizing agent gas channel. Then, hydrogen supplied to the anode is protonated on the catalyst layer and the generated protons move to the cathode through the electrolyte membrane. At this time, electrons generated together with protons are taken to an external circuit and used as electric energy.
In another type of a known membrane electrode assembly, the planar size of the diffusion layer of one electrode is smaller than that of the diffusion layer of the other electrode, and thereby, a step is formed in the outer peripheral edge. In this membrane electrode assembly, the planar size of the electrolyte membrane is equal to that of the diffusion layer of the other electrode, and thus, the electrolyte membrane in the outer peripheral edge is not held between these diffusion layers. In view of this, to protect the assembly mechanically and chemically, a configuration in which the outer peripheral edge of an exposed part of the electrolyte membrane is protected with a resin frame of, for example, a resin molding element and a resin film is proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2007-66766).
In this configuration, when an inner edge of the resin frame is disposed between the electrolyte membrane and the diffusion layer, a load is concentrated on the inner edge of the resin frame because of a contact pressure generally applied to the membrane electrode assembly. Consequently, the electrolyte membrane is pressed and has its thickness reduced, resulting in deterioration of power generation performance. To prevent this deterioration, the inner edge of the resin frame needs to be located outside the diffusion layer. In this case, however, a clearance occurs between the inner edge of the resin frame and the outer edge of the diffusion layer so that the exposed part of the outer peripheral edge of the electrolyte membrane cannot be sufficiently protected. Thus, a configuration in which a filler such as an adhesive is provided in the clearance between the inner edge of the resin frame and the outer edge of the diffusion layer has been employed.